Detention and surge facilities temporarily store a fluid and limit the rate of fluid discharge to a downstream system when the inflow rate of the fluid is variable and at times exceeds the functional capacity of the downstream system. In the case of a storm water detention system, increased volumes of storm water runoff, typically generated by the development of upstream lands, flow into the system at varying rates and are temporarily stored for delayed release downstream to reduce incidences of flooding, harmful erosion, rapid fluctuations in water temperature and other environmental consequences which occur when receiving bodies of water such as lakes, rivers, streams and oceans receive flow at significantly increased rates. Similarly, a surge tank temporarily stores a process fluid of varying inflow rate and limits the rate of discharge of the fluid to that which will not exceed the capacity of a downstream process. In the field of wastewater treatment, a surge tank is sometimes deployed to receive wastewater flows during peak periods of water use. The surge tank temporarily stores the wastewater and limits the release of the wastewater flow to the treatment plant to a rate not exceeding the design capacity of the plant.
The temporary storage volume required for a detention system or surge tank is dependent on the rate and duration of fluid inflow and the allowable rate and duration of fluid outflow. The larger the difference between the peak rate of inflow and the allowable rate of outflow, the greater the volume is required for temporary storage.
Providing large storage volumes can be costly including, for example, the expense incurred for land acquisition and excavation required to construct a large detention pond or the expense of fabrication and installation of a very large tank. It is therefore advantageous to minimize the amount of temporary storage volume required. Minimization of the required temporary storage volume can be accomplished by minimizing the difference between the duration and rate of inflow and the duration and rate of outflow. Since the rate of inflow is variable and cannot be controlled, minimization of the required temporary storage volume is achieved when the maximum allowable rate of discharge is achieved at the earliest point in time following the commencement of inflow and is sustained for the longest possible duration of time.
The prior art is generally concerned with limiting the maximum outflow rates, at which damage can occur, by employing discharge control mechanisms such as fixed weirs, orifices, nozzles and riser structures whereby the maximum discharge rates of such mechanisms are determined by the geometric configuration of the mechanisms and the height of the fluid (static head) acting on the mechanisms. In each case, the maximum flow rate is achieved only at the single point in time at which the static head acting on the mechanism is at its maximum level. Therefore, all discharges occurring when fluid levels are not at their maximums are less than optimal.
One solution to this problem is described in U.S. Pat. No. 7,125,200 to Fulton, which is hereby incorporated by reference. This patent describes a flow control device that consists of a buoyant flow control module housing an orifice within an interior chamber that is maintained at a predetermined depth below the water surface. This flow control device neglects the use of other traditional flow control mechanisms such as weirs, risers and nozzles, has some adjustability, and utilizes flexible moving parts.
Stormwater runoff is often laden with a variety of pollutants from a variety of sources such as oils and greases from roadways and parking lots, nitrogen and phosphorous from agricultural and horticultural pursuits, heavy metals (e.g. lead, copper, zinc, chromium) from industrial activities, and suspended solids such as silt and clay particles which tend to erode at accelerated rates when lands are stripped of vegetation during construction activities. Because stormwater runoff is so often polluted, stormwater management systems typically include a component process called a Best Management Practice (BMP) which is intended to reduce the quantity of pollution discharged to receiving bodies of water such as lakes, rivers, streams and oceans. In the case of a stormwater detention system, the BMP component is most often located at the bottom of the system, below the volume of storage required to mitigate increased discharge rates which may cause flooding, erosion, rapid fluctuations in water temperature and other environmental consequences (the minimum elevation at which water may be released). One such BMP is known as retention, wherein a certain volume of stormwater runoff, often referred to as the “first flush” is collected and stored between the bottom elevation of the system and the minimum elevation at which water may be released. The “first flush” is typically disposed of through a combination of the processes of infiltration and evapotranspiration. Another such BMP is known as “extended detention”, wherein the “first flush” of runoff is also collected and stored between the bottom of the system and the minimum elevation at which water may be released; however, the “first flush” is slowly drained to the receiving body of water over a predetermined period of time (typically 24 to 72 hours) allowing for physical, chemical and biological processes such as sedimentation, oxidation, precipitation, adsorption and denitrification to reduce the quantity of pollution contained in the discharge. In the stormwater treatment industry, the volume of the “first flush” is not uniformly defined and often varies geographically. The “first flush” volume is most often related to the depth of rainfall for events which recur annually at a frequency equal to the desired level of treatment. For example, in a certain geographic region, 95% of all storm events deliver a total rainfall depth of one inch or less. Therefore, if the volume of the “first flush” is treated with the BMP of retention and the volume treated is equivalent to the amount of runoff generated from the first inch of rainfall, then at least 95% of all pollution will be removed from the discharge on an annual average basis. Similarly, it follows that if a volume greater than the runoff generated by the first inch of rainfall is retained in the system, then more than 95% of all pollution will be removed from the discharge on an average annual basis. Likewise, in the same geographic region, if the volume of the “first flush” is treated with the BMP of “extended detention” and the “extended detention” process is known or presumed to remove 80% of the total mass of pollutants, than at least 76% (80%×95%=76%) of all pollution will be removed from the discharge on an annual average basis. Thus, greater masses of pollution can be removed from stormwater discharges if larger BMP volumes are provided in the stormwater management system.
What is needed is a flow control device that allows for greater BMP volumes of storm water to be accommodated in stormwater management systems which also provide a certain amount of storage volume above the BMP volume for the purposes of reducing incidences of flooding, harmful erosion, rapid fluctuations in temperature and other environmental consequences which result when a body of water such as a lake, river, stream or ocean receives increased volumes of stormwater discharges at increased rates of inflow.